Material for lithium secondary battery of high performance

ABSTRACT

Provided is a lithium mixed transition metal oxide having a composition represented by Formula I of Li x M y O 2  (M, x and y are as defined in the specification)having mixed transition metal oxide layers (“MO layers”) comprising Ni ions and lithium ions, wherein lithium ions intercalate into and deintercalate from the MO layers and a portion of MO layer-derived Ni ions are inserted into intercalation/deintercalation layers of lithium ions (“reversible lithium layers”) thereby resulting in the interconnection between the MO layers. The lithium mixed transition metal oxide of the present invention has a stable layered structure and therefore exhibits improved stability of the crystal structure upon charge/discharge. In addition, a battery comprising such a cathode active material can exhibit a high capacity and a high cycle stability. Further, such a lithium mixed transition metal oxide is substantially free of water-soluble bases, and thereby can provide excellent storage stability, decreased gas evolution and consequently superior high-temperature stability with the feasibility of low-cost mass production.

FIELD OF THE INVENTION

The present invention relates to a Ni-based lithium mixed transitionmetal oxide and a cathode active material for a secondary batterycomprising the same. More specifically, the Ni-based lithium mixedtransition metal oxide according to the present invention has a givencomposition and exhibits intercalation/deintercalation of lithium ionsinto/from mixed transition metal oxide layers (“MO layers”) andinterconnection of MO layers via the insertion of a portion of MOlayer-derived Ni ions into intercalation/deintercalation layers(reversible lithium layers) of lithium ions, thereby improving thestructural stability of the crystal structure upon charge/discharge toprovide an excellent sintering stability. In addition, a batterycomprising such a cathode active material can exert a high capacity anda high cycle stability. Further, with substantially no water-solublebases present, such a lithium mixed transition metal oxide exhibitsexcellent storage stability and chemical resistance, and decreased gasevolution, thereby resulting in an excellent high-temperature stabilityand the feasibility of mass production at low cost.

BACKGROUND OF THE INVENTION

Technological development and increased demand for mobile equipment haveled to a rapid increase in the demand for secondary batteries as anenergy source. Among other things, lithium secondary batteries having ahigh-energy density and voltage, a long cycle lifespan and a lowself-discharge rate are commercially available and widely used.

As cathode active materials for the lithium secondary batteries,lithium-containing cobalt oxide (LiCoO₂) is largely used. In addition,consideration has been made to using lithium-containing manganese oxidessuch as LiMnO₂ having a layered crystal structure and LiMn₂O₄ having aspinel crystal structure, and lithium-containing nickel oxides (LiNiO₂).

Of the aforementioned cathode active materials, LiCoO₂ is currentlywidely used due to superior general properties including excellent cyclecharacteristics, but suffers from low safety, expensiveness due tofinite resources of cobalt as a raw material, and limitations inpractical and mass application thereof as a power source for electricvehicles (EVs) and the like.

Lithium manganese oxides, such as LiMnO₂ and LiMn₂O₄, are abundantresources as raw materials and advantageously employenvironmentally-friendly manganese, and therefore have attracted a greatdeal of attention as a cathode active material capable of substitutingLiCoO₂. However, these lithium manganese oxides suffer from shortcomingssuch as low capacity and poor cycle characteristics.

Whereas, lithium/nickel-based oxides including LiNiO₂ are inexpensive ascompared to the aforementioned cobalt-based oxides and exhibit a highdischarge capacity upon charging to 4.3 V. The reversible capacity ofdoped LiNiO₂ approximates about 200 mAh/g which exceeds the capacity ofLiCoO₂ (about 165 mAh/g). Therefore, despite a slightly lower averagedischarge voltage and a slightly lower volumetric density, commercialbatteries comprising LiNiO₂ as the cathode active material exhibit animproved energy density. To this end, a great deal of intensive researchis being actively undertaken on the feasibility of applications of suchnickel-based cathode active materials for the development ofhigh-capacity batteries. However, the LiNiO₂-based cathode activematerials suffer from some limitations in practical application thereof,due to the following problems.

First, LiNiO₂-based oxides undergo sharp phase transition of the crystalstructure with volumetric changes accompanied by repeatedcharge/discharge cycling, and thereby may suffer from cracking ofparticles or formation of voids in grain boundaries. Consequently,intercalation/deintercalation of lithium ions may be hindered toincrease the polarization resistance, thereby resulting in deteriorationof the charge/discharge performance. In order to prevent such problems,conventional prior arts attempted to prepare a LiNiO₂-based oxide byadding an excess of a Li source and reacting reaction components underan oxygen atmosphere. However, the thus-prepared cathode activematerial, under the charged state, undergoes structural swelling anddestabilization due to the repulsive force between oxygen atoms, andsuffers from problems of severe deterioration in cycle characteristicsdue to repeated charge/discharge cycles.

Second, LiNiO₂ has shortcomings associated with evolution of excess ofgas during storage or cycling. That is, in order to smoothly form thecrystal structure, an excess of a Li source is added duringmanufacturing of the LiNiO₂-based oxide, followed by heat treatment. Asa result, water-soluble bases including Li₂CO₃ and LiOH reactionresidues remain between primary particles and thereby they decompose orreact with electrolytes to thereby produce CO₂ gas, upon charging.Further, LiNiO₂ particles have an agglomerate secondary particlestructure in which primary particles are agglomerated to form secondaryparticles and consequently a contact area with the electrolyte furtherincreases to result in severe evolution of CO₂ gas, which in turnunfortunately leads to the occurrence of battery swelling anddeterioration of desirable high-temperature safety.

Third, LiNiO₂ suffers from a sharp decrease in the chemical resistanceof a surface thereof upon exposure to air and moisture, and gelation ofslurries by polymerization of an N-methyl pyrrolidone/poly(vinylidenefluoride) (NMP-PVDF) slurry due to a high pH value. These properties ofLiNiO₂ cause severe processing problems during battery production.

Fourth, high-quality LiNiO₂ cannot be produced by a simple solid-statereaction as is used in the production of LiCoO₂, and LiNiMeO₂ (Me=Co, Mnor Al) cathode active materials containing an essential dopant cobaltand further dopants manganese and aluminum are produced by reacting alithium source such as LiOH.H₂O with a mixed transition metal hydroxideunder an oxygen or syngas atmosphere (i.e., a CO₂-deficient atmosphere),which consequently increases production costs. Further, when anadditional step, such as intermediary washing or coating, is included toremove impurities in the production of LiNiO₂, this leads to a furtherincrease in production costs.

Further, Japanese Unexamined Patent Publication Nos. 2004-281253,2005-150057 and 2005-310744 disclose oxides having a composition formulaof Li_(a)Mn_(x)Ni_(y)M_(z)O₂ (M=Co or Al, 1≦a≦1.2, 0≦x≦0.65, 0.35≦y≦1,0≦z≦0.65, and x+y+z=1). However, it was found through variousexperiments conducted by the inventors of the present invention that theaforementioned oxides include large amounts of impurities such aslithium carbonates, and suffer from significant problems associated withsevere gas evolution at high temperatures and structural instability.

Therefore, there is an urgent need in the art for the development of atechnology which is capable of achieving a structurally stable crystalstructure while utilizing a lithium/nickel-based cathode active materialwhich can allow for high charge capacity and is capable of securinghigh-temperature safety.

SUMMARY OF THE INVENTION

Therefore, the present invention has been made to solve the aboveproblems and other technical problems that have yet to be resolved.

As a result of a variety of extensive and intensive studies andexperiments and in view of the problems as described above, theinventors of the present invention provide herewith a lithium mixedtransition metal oxide, as will be illustrated hereinafter, having agiven composition and a specific atomic-level structure, with which itis possible to realize superior thermal stability, and high cyclestability in conjunction with a high capacity, due to improvements inthe stability of the crystal structure upon charge/discharge. Further,such a lithium mixed transition metal oxide can be prepared in asubstantially water-soluble base-free form and therefore exhibitsexcellent storage stability, reduced gas evolution and thereby excellenthigh-temperature safety in conjunction with the feasibility ofindustrial-scale production at low production costs. The presentinvention has been completed based on these findings.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill be more clearly understood from the following detailed descriptiontaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic view showing a crystal structure of a conventionalNi-based lithium transition metal oxide;

FIG. 2 is a schematic view showing a crystal structure of a Ni-basedlithium mixed transition metal oxide according to an embodiment;

FIGS. 3 and 4 are graphs showing a preferred composition range of aNi-based lithium mixed transition metal oxide according to anembodiment;

FIG. 5 is an FESEM (Field Emission Scanning Electron Microscope) image(×2,000) showing LiNiMO₂ according to Example 1 of the presentinvention. 5A: 850° C.; 5B: 900° C.; 5C: 950° C.; and 5D: 1000° C.;

FIG. 6 is an FESEM image showing commercial LiMO₂ (M=Ni_(0.8)Co_(0.2))according to Comparative Example 1. 6A: FESEM image of a sample asreceived, and 6B: FESEM image of a sample after heating to 850° C. inair;

FIG. 7 is an FESEM image showing the standard pH titration curve ofcommercial high-Ni LiNiO₂ according to Comparative Example 2. A: Sampleas received, B: After heating of a sample to 800° C. under an oxygenatmosphere, and C: Copy of A;

FIG. 8 is a graph showing a pH titration curve of a sample according toComparative Example 3 during storage of the sample in a wet chamber. A:Sample as received, B: After storage of a sample for 17 hrs, and C:After storage of a sample for 3 days;

FIG. 9 is a graph showing a pH titration curve of a sample according toExample 2 during storage of the sample in a wet chamber. A: Sample asreceived, B: After storage of a sample for 17 hrs, and C: After storageof a sample for 3 days;

FIG. 10 is an SEM image of a sample according to Example 3;

FIG. 11 shows the Rietveld refinement on X-ray diffraction patterns of asample according to Example 3;

FIG. 12 is a graph showing electrochemical properties of LiNiMO₂according to Example 3 in Experimental Example 1. 12A: Graph showingvoltage profiles and rate characteristics at room temperature (1 to 7cycles); 7B: Graph showing cycle stability at 25° C. and 60° C. and arate of C/5 (3.0 to 4.3V); and 7C: Graph showing discharge profiles (atC/10 rate) for Cycle 2 and Cycle 31, obtained during cycling at 25° C.and 60° C.;

FIG. 13 is a graph showing DSC (differential scanning calorimetry)values for samples of Comparative Examples 3 and 4 in ExperimentalExample 2. A: Commercial Al/Ba-modified LiMO₂ (M=Ni) of ComparativeExample 3, and B: Commercial AlPO₄-coated LiMO₂ (M=Ni) of ComparativeExample 4;

FIG. 14 is a graph showing DSC values for LiNiMO₂ according to Example 3in Experimental Example 2;

FIG. 15 is a graph showing electrophysical properties of a polymer cellaccording to one embodiment in Experimental Example 3;

FIG. 16 is a graph showing swelling of a polymer cell duringhigh-temperature storage in Experimental Example 3; and

FIG. 17 is a graph showing lengths of a-axis and c-axis ofcrystallographic unit cells of samples having different molar ratios ofLi:M in Experimental Example 4.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In accordance with an aspect of the present invention, the above can beaccomplished by the provision of a lithium mixed transition metal oxidehaving a composition represented by Formula I below, wherein lithiumions undergo intercalation/deintercalation into/fromintercalation/deintercalation layers which are disposed alternately andrepeatedly with mixed transition metal oxide layers (“MO layers”)containing Ni ions, and a portion of MO layer-derived Ni ions areinserted into the intercalation/deintercalation layers of lithium ions(also referred to herein as “reversible lithium layers”) therebyresulting in the interconnection of and between the MO layers andreversible lithium layers.

Li_(x)M_(y)O₂   (I)

wherein:

M=M′_(1−k)A_(k), wherein M′ is Ni_(1−a−b)(N_(1/2)Mn_(1/2))_(a)Co_(b),0.65≦a+b≦0.85 and 0.1≦b≦0.4;

A is a dopant;

0≦k≦0.05; and

x+y≈2 and 0.95≦x≦1.05.

A conventional lithium transition metal oxide has a layered crystalstructure as shown in FIG. 1, and performs charge/discharge processesthrough lithium insertion and desertion into/from the reversible lithiumlayers. However, when the oxide having such a structure is used as acathode active material, deintercalation of lithium ions from thereversible lithium layers in the charged state brings about swelling anddestabilization of the crystal structure due to the repulsive forcebetween oxygen atoms in the MO layers, and thus lithium transition metaloxide suffers from problems associated with sharp decreases in thecapacity and cycle characteristics, resulting from the structuralchanges in the crystal structure due to repeated charge/dischargecycles.

As a result of a variety of extensive and intensive studies andexperiments to solve the problems as described above, the inventors ofthe present invention have surprisingly found that the crystal structurein the lithium mixed transition metal oxides disclosed herein stabilizescontrary to conventionally known or accepted ideas in the related artthat intercalation/deintercalation of lithium ions will be hindered whena portion of the nickel ions present transfer to and are immobilized inthe reversible lithium layers as shown in FIG. 2, so it is possible toprevent problems associated with collapse of the crystal structurecaused by the intercalation/deintercalation of lithium ions. As aresult, since the lifespan characteristics and safety of the lithiummixed transition metal oxide are simultaneously improved due to a lackof occurrence of additional structural collapse by oxygen desorption andprevention of further formation of Ni²⁺ ions, the battery capacity andcycle characteristics can be significantly improved and a desired levelof rate characteristics can be exerted. Therefore, it can be said thatsuch a concept of the present invention is a remarkable one which iscompletely opposite to and overthrows the conventional ideas ofstructural stability in such systems.

That is, due to the insertion of a portion of Ni ions into thereversible lithium layers, the lithium mixed transition metal oxide inaccordance with the present invention does not undergo disintegration ofthe crystal structure with maintenance of the oxidation number of Niions inserted into the reversible lithium layers, even when lithium ionsare released during a charge process, thereby being capable ofmaintaining a well-layered structure. Hence, a battery comprising thelithium mixed transition metal oxide having such a structure as acathode active material can exert a high capacity and a high-cyclestability.

Further, the lithium mixed transition metal oxide in accordance with thepresent invention has excellent thermal stability as the crystalstructure is stably maintained even upon sintering at a relatively hightemperature during a production process.

Generally, when conventional high-nickel LiMO₂ is subjected tohigh-temperature sintering in air containing a trace amount of CO₂,LiMO₂ decomposes with a decrease of Ni³⁺ ions as shown in the followingreaction below, and during which amounts of impurities Li₂CO₃ increase.

LiM³⁺O₂+CO₂→a Li_(1−x)M_(1+x) ^(3+,2+)O₂+b Li₂CO₃+c O₂

Further, continuous degradation of the crystal structure occurs withincreased cation mixing, an increased lattice constant and a decreasedc:a ratio, molten Li₂CO₃ impurities segregate particles of the LiMO₂,and primary particles lose a contact state therebetween, therebyresulting in the disintegration of secondary particles.

However, unlike the conventional high-nickel LiMO₂, the lithium mixedtransition metal oxide in accordance with the present invention, due tothe stability of the atomic-level structure, does not include Li₂CO₃impurities resulting from oxygen deficiency due to a decrease in anddecomposition of Ni³⁺ ions as shown in the above reaction, and thereforeexhibits no degradation of the crystal structure, even when subjected tohigh-temperature sintering under an air atmosphere. Further, there aresubstantially no water-soluble bases present, as the lithium mixedtransition metal oxide of the present invention can be prepared withoutaddition of an excess of a lithium source. Accordingly, the lithiummixed transition metal oxide of the present invention exhibits excellentstorage stability, decreased gas evolution and thereby excellenthigh-temperature stability simultaneously with the feasibility ofindustrial-scale production at low production costs.

In contrast to the desirable properties of the lithium mixed transitionmetal oxide of the present invention, the inventors hereof have foundthat the above-referenced oxides disclosed in Japanese Unexamined PatentPublication Nos. 2004-281253, 2005-150057 and 2005-310744, and whichhave a similar composition, nonetheless do not have the same structureas that of the lithium mixed transition metal oxides of the presentinvention, include large amounts of impurities such as lithiumcarbonates, and suffer from severe gas evolution at high temperaturesand structural instability.

Hereinafter, where appropriate throughout the specification, the term“lithium mixed transition metal oxide in accordance with the presentinvention” is used interchangeably with the term “LiNiMO₂”, and the term“Ni ions inserted and bound into the reversible lithium layers” is usedinterchangeably with the term “inserted Ni”. Therefore, NiM in LiNiMO₂is a suggestive expression representing a complex composition of Ni, Mnand Co in Formula I.

In one specific embodiment, the lithium mixed transition metal oxidehave a structure wherein Ni²⁺ and Ni³⁺ ions ons coexist in the MO layersand a portion of Ni²⁺ ions are inserted into the reversible lithiumlayers. That is, in such a structure of the metal oxide, all of Ni ionsinserted into the reversible lithium layers are Ni²⁺ ions and theoxidation number of Ni ions is not changed in the charge process.

Specifically, when Ni²⁺ and Ni³⁺ ions coexist in a Ni-excess lithiumtransition metal oxide, an oxygen atom-deficient state is present undergiven conditions (reaction atmosphere, Li content, and the like) andtherefore insertion of a portion of the Ni²⁺ ions into the reversiblelithium layers may occur with changes in the oxidation number of Ni.

The composition of the lithium mixed transition metal oxide shouldsatisfy the following specific requirements as defined in Formula I:

(i) Ni_(1−(a+b)()N_(1/2)Mn_(1/2))_(a)Co_(b) and 0.65≦a+b≦0.85

(ii) 0.1≦b≦0.4, and

(iii) x+y≈2 and 0.95≦x≦1.05

Regarding the aforementioned requirement (i), Ni_(1−(a+b)) means acontent of ions of Ni³⁺. Therefore, if a mole fraction of Ni³⁺ exceeds0.35 (a+b<0.65), it is impossible to implement an industrial-scaleproduction in air, using Li₂CO₃ as a precursor material, so the lithiumtransition metal oxide should be produced using LiOH as a precursormaterial under an oxygen atmosphere, thereby presenting a problemsassociated with decreased production efficiency and consequentlyincreased production costs. On the other hand, if a mole fraction ofNi³⁺ is lower than 0.15 (a+b>0.85), the capacity per volume of LiNiMO₂is not competitive as compared to LiCoO₂.

Meanwhile, taking into consideration both of the above requirements (i)and (ii), the total mole fraction of Ni ions including Ni²⁺ ions andNi³⁺ ions in LiNiMO₂ of the present invention is preferably of arelative nickel-excess compared to manganese and cobalt and may be 0.4to 0.7. If a content of nickel is excessively low, it is difficult toachieve a high capacity. Conversely, if a content of nickel isexcessively high, the safety may be significantly lowered. Inconclusion, the lithium transition metal oxide (LiNiMO₂) exhibits alarge volume capacity and low raw material costs, as compared to lithiumcobalt-based oxides.

Further, if the mole fraction of Ni²⁺ ions is too high relative to theNi content, the cation mixing increases, resulting in formation of a“rock salt” like crystalline structure that is excessively stable to thepoint of being locally and electrochemically non-reactive, and such arock salt structure not only hinders charge/discharge and but also maybring about a decrease in a discharge capacity. On the other hand, ifthe mole fraction of Ni²⁺ ions is too low, this may lead to an increasein the structural instability to thereby lower the cycle stability.Therefore, the mole fraction of Ni²⁺ ions should be appropriatelyadjusted taking into consideration such problems that may occur.Preferably, within the range as shown in FIG. 3, the mole fraction ofNi²⁺ ions may be 0.05 to 04, based on the total content of Ni.

Therefore, since Ni²⁺ is inserted into the reversible lithium layers andserves to support the MO layers, Ni²⁺ is contained in an amountsufficient to provide a stablestructural support between MO layers suchthat the charge stability and cycle stability can be improved to adesired level, and at the same time it is inserted in an amount not soas to hinder intercalation/deintercalation of lithium ions into/from thereversible lithium layers such that rate characteristics are notdeteriorated. Taken altogether, the mole fraction of Ni²⁺ ions insertedand bound into the reversible lithium layers may be specifically 0.03 to0.07, based on the total molar content of Ni.

The content of Ni²⁺ ions or the content of inserted Ni²⁺ ions may bedetermined by controlling, for example, a sintering atmosphere, thecontent of lithium, and the like. For example, when a concentration ofoxygen in the sintering atmosphere is high, the content of Ni²⁺ will berelatively low.

With regard to the aforementioned condition (ii), a content of cobalt(b) is 0.1 to 0.4. If the content of cobalt is excessively high (b>0.4),the overall cost of a raw material increases due to a high content ofcobalt, and the reversible capacity decreases. On the other hand, if thecontent of cobalt is excessively low (b<0.1), it is difficult to achievesufficient rate characteristics and a high powder density of the batteryat the same time.

With regard to the aforementioned condition (iii), if a content oflithium is excessively high, i.e. x>1.05, this may result in a problemof decreased stability during charge/discharge cycling, particularly atT=60° C. and a high voltage (U=4.35 V). On the other hand, if a contentof lithium is excessively low, i.e. x<0.95, this may result in poor ratecharacteristics and a decreased reversible capacity.

In an embodiment, LiNiMO₂ may further comprise trace amounts of dopants.Examples of the dopants may include aluminum, titanium, magnesium andthe like, which are incorporated into the crystal structure. Further,other dopants, such as B, Ca, Zr, F, P, Bi, Al, Mg, Zn, Sr, Ga, In, Ge,and Sn, may be included via the grain boundary accumulation or surfacecoating of the dopants without being incorporated into the crystalstructure. These dopants are included in amounts enough to increase thesafety, capacity and overcharge stability of the battery while notcausing a significant decrease in the reversible capacity. Therefore, acontent of the dopant is less than 5% (k<0.05), as defined in Formula I.In addition, the dopants may be preferably added in an amount of <1%,within a range that can improve the stability without causingdeterioration of the reversible capacity.

As a molar ratio (Li/M) of Li to the transition metal (M) decreases, theamount of Ni ions present in the MO layers gradually increases.Therefore, if excessive amounts of Ni ions transfer into the reversiblelithium layers, a movement of Li⁺ during charge/discharge processes ishampered to thereby lead to problems associated with a decrease in thereversible capacity or deterioration of the rate characteristics. On theother hand, the Li/M ratio is excessively high, the amount of Ni presentin the MO layer is excessively low, which may undesirably lead tostructural instability, thereby presenting decreased safety of thebattery and poor lifespan characteristics. Further, at an excessivelyhigh Li/M value, amounts of unreacted Li₂CO₃ increase to thereby resultin a high pH-titration value, i.e., production of large amounts ofimpurities, and consequently the chemical resistance andhigh-temperature stability may deteriorate. Therefore, in one preferredembodiment, the ratio of Li:M in LiNiMO₂ may be 0.95 to 1.04:1.

In one embodiment, the lithium mixed transition metal oxide inaccordance with the present invention is substantially free ofwater-soluble base impurities, particularly Li₂CO₃.

Usually, Ni-based lithium transition metal oxides contain large amountsof water-soluble bases such as lithium oxides, lithium sulfates, lithiumcarbonates, and the like. These water-soluble bases may be bases, suchas Li₂CO₃ and LiOH, present in LiNiMO₂, or otherwise may be basesproduced by ion exchange (H⁺ (water) ←→ Li⁺ (surface, an outer surfaceof the bulk)), performed at the surface of LiNiMO₂. The bases of thelatter case are usually present at a negligible level. The formerwater-soluble bases are formed primarily due to unreacted lithium rawmaterials upon sintering. This is because as a conventional Ni-basedlithium transition metal oxide employs relatively large amounts oflithium and low-temperature sintering so as to prevent the collapse of alayered crystal structure, the resulting metal oxide has relativelylarge amounts of grain boundaries as compared to the cobalt-based oxidesand lithium ions do not sufficiently react upon sintering.

On the other hand, since LiNiMO₂ in accordance with the presentinvention, as discussed hereinbefore, maintains a structurally stablelayered crystal structure, it is possible to carry out a sinteringprocess at a relatively high-temperature under an air atmosphere andthereby LiNiMO₂ has relatively small amounts of grain boundaries. Inaddition, as retention of unreacted lithium species on the surfaces ofparticles is prevented, the particle surfaces are substantially free oflithium salts such as lithium carbonates, lithium sulfates, and thelike. Further, there is no need to add an excess of a lithium sourceupon production of LiNiMO₂, so it is possible to fundamentally prevent aproblem associated with the formation of impurities due to the unreactedlithium source remaining in the powder.

As such, it is possible to fundamentally solve various problems that mayoccur due to the presence of the water-soluble bases, particularly theproblem that may damage the battery safety due to evolution of gasarising from acceleration of electrolyte decomposition at hightemperatures.

As used herein, the phrase “is (are) substantially free of water-solublebases” refers to an extent that upon titration of 200 mL of a solutioncontaining the lithium mixed transition metal oxide with 0.1M HCl, theamount of HCl solution used to reach a pH of less than 5 is less than 20mL, more preferably less than 10 mL. Herein, 200 mL of theaforementioned solution contains substantially all of the water-solublebases in the lithium mixed transition metal oxide, and is prepared byrepeatedly soaking and decanting 10 g of the lithium mixed transitionmetal oxide.

More specifically, first, 5 g of a cathode active material powder isadded to 25 mL of water, followed by brief stirring. About 20 mL of aclear solution is separated and pooled from the powder by soaking anddecanting. Again, about 20 mL of water is added to the powder and theresulting mixture is stirred, followed by decanting and pooling. Thesoaking and decanting are repeated at least 5 times. In this manner,total 100 mL of the clear solution containing water-soluble bases ispooled. A 0.1M HCl solution is added to titrate the thus-pooledsolution, where pH titration is done with stirring. The pH profile isrecorded as a function of time. Experiments are terminated when the pHreaches a value of less than about 3, and a flow rate may be selectedwithin a range that titration takes about 20 to about 30 min. Thecontent of the water-soluble bases is given as an amount of acid thatwas used until the pH reaches a value of less than about 5.

In this manner, it is possible to determine the content of thewater-soluble bases contained in the aforesaid powder. At this time,there are no significant influences of parameters such as a totalsoaking time of the powder in water.

A method of preparing the lithium mixed transition metal oxide inaccordance with the present invention may be preferably carried outunder an oxygen-deficient atmosphere. The oxygen-deficient atmospheremay be an atmosphere with an oxygen concentration of preferably 10% to50% by volume, more preferably 10% to 30% by volume. In an embodiment,the lithium mixed transition metal oxide can be prepared by asolid-state reaction of Li₂CO₃ and mixed transition metal precursors inthe air atmosphere. Therefore, it is also possible to solve variousproblems associated with increased production costs and the presence oflarge amounts of water-soluble bases, which the conventional arts sufferfrom when making the lithium transition metal oxide involving adding,mixing and sintering of LiOH and each transition metal precursor underan oxygen atmosphere. That is, the present invention enables productionof the lithium mixed transition metal oxide having a given compositionand a high content of nickel via a simple solid-state reaction in air,using a raw material that is cheap and easy to handle. Therefore, it ispossible to realize a significant reduction of production costs andhigh-efficiency mass production.

That is, due to thermodynamic limitations in the conventional art, itwas impossible to produce high-nickel lithium mixed transition metaloxide in the air containing trace amounts of carbon dioxide. Inaddition, the conventional art suffered from problems in thatutilization of Li₂CO₃ as a precursor material brings about evolution ofCO₂ due to decomposition of Li₂CO₃, which then thermodynamically hindersadditional decomposition of Li₂CO₃ even at a low partial pressure,consequently resulting in no further progression of the reaction. Forthese reasons, it was impossible to use Li₂CO₃ as the precursor in thepractical production process.

On the other hand, the lithium mixed transition metal oxide inaccordance with the present invention employs Li₂CO₃ and the mixedtransition metal precursor as the reaction materials, and can beprepared by reacting reactants in an oxygen-deficient atmosphere,preferably in air. As a result, it is possible to achieve a significantreduction of production costs, and a significant increase of theproduction efficiency due to the feasibility to produce a desiredproduct by a relatively simple process. This is because the lithiummixed transition metal oxide in accordance with the present invention isvery excellent in the sintering stability at high temperatures, i.e. thethermodynamic stability, due to a stable crystal structure.

Further, since the lithium source material and the mixed transitionmetal material may be preferably added in a ratio of 0.95 to 1.04:1 whenproducing the lithium mixed transition metal oxide in accordance withthe present invention, there is no need to add an excess of a lithiumsource and it is possible to significantly reduce the possibility ofretention of the water-soluble bases which may derive from the residuallithium source.

In order to carry out a smooth reaction via high air circulation uponproduction of the lithium mixed transition metal oxide by thelarge-scale mass production process, preferably at least 2 m³ (a volumeat room temperature) of air, per kg of the final lithium mixedtransition metal oxide, may be pumped into or out of a reaction vessel.For this purpose, a heat exchanger may be employed to further enhancethe efficiency of energy utilization by pre-warming the in-flowing airbefore it enters the reactor, while cooling the out-flowing air.

In accordance with a further aspect of the present invention, there isprovided a cathode active material for a secondary battery comprisingthe aforementioned lithium nickel-based oxide.

The cathode active material in accordance with the present invention maybe comprised only of the lithium mixed transition metal oxide having theabove-specified composition and the specific atomic-level structure, orwhere appropriate, it may be comprised of the lithium mixed transitionmetal oxide in conjunction with other lithium-containing transitionmetal oxides.

Examples of the lithium-containing transition metal oxides that can beused in the present invention may include, but are not limited to,layered compounds such as lithium cobalt oxide (LiCoO₂) and lithiumnickel oxide (LiNiO₂), or compounds substituted with one or moretransition metals; lithium manganese oxides such as compounds of FormulaLi_(1+y)Mn_(2-y)O₄ (0≦y≦0.33), LiMnO₃, LiMn₂O₃, and LiMnO₂; lithiumcopper oxide (Li₂CuO₂); vanadium oxides such as LiV₃O₈, V₂O₅, andCu₂V₂O₇; Ni-site type lithium nickel oxides of Formula LiNi_(1−y)M_(y)O₂(M=Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and 0.01≦y≦0.3); lithium manganesecomposite oxides of Formula LiMn_(2-y)M_(y)O₂ (M=Co, Ni, Fe, Cr, Zn, orTa, and 0.01≦y≦0.1), or Formula Li₂Mn₃MO₈ (M=Fe, Co, Ni, Cu, or Zn);LiMn₂O₄ wherein a portion of Li is substituted with alkaline earth metalions; disulfide compounds; and Fe₂(MoO₄)₃, LiFe₃O₄, and the like.

In accordance with a still further aspect of the present invention,there is provided a lithium secondary battery comprising theaforementioned cathode active material. The lithium secondary battery isgenerally comprised of a cathode, an anode, a separator and a lithiumsalt-containing non-aqueous electrolyte. Methods for preparing thelithium secondary battery are known and therefore detailed descriptionthereof will be omitted herein.

The cathode is, for example, fabricated by applying a mixture of theabove-mentioned cathode active material, a conductive material and abinder to a cathode current collector, followed by drying. If necessary,a filler may be further added to the above mixture.

The cathode current collector is generally fabricated to have athickness of 3 to 500 μm. There is no particular limit to materials forthe cathode current collector, so long as they have high conductivitywithout causing chemical changes in the fabricated battery. As examplesof the materials for the cathode current collector, mention may be madeof stainless steel, aluminum, nickel, titanium, sintered carbon, andaluminum or stainless steel which was surface-treated with carbon,nickel, titanium or silver. The current collector may be fabricated tohave fine irregularities on the surface thereof so as to enhanceadhesion to the cathode active material. In addition, the currentcollector may take various forms including films, sheets, foils, nets,porous structures, foams and non-woven fabrics.

The conductive material is typically added in an amount of 1 to 50% byweight, based on the total weight of the mixture including the cathodeactive material. There is no particular limit to the conductivematerial, so long as it has suitable conductivity without causingchemical changes in the fabricated battery. As examples of conductivematerials, mention may be made of conductive materials, includinggraphite such as natural or artificial graphite; carbon black such ascarbon black, acetylene black, Ketjen black, channel black, furnaceblack, lamp black and thermal black; conductive fibers such as carbonfibers and metallic fibers; metallic powders such as carbon fluoridepowder, aluminum powder and nickel powder; conductive whiskers such aszinc oxide and potassium titanate; conductive metal oxides such astitanium oxide; and polyphenylene derivatives.

The binder is a component assisting in binding between the electrodeactive material and the conductive material, and in binding of theelectrode active material to the current collector. The binder istypically added in an amount of 1 to 50% by weight, based on the totalweight of the mixture including the cathode active material. As examplesof the binder, mention may be made of polyvinylidene fluoride, polyvinylalcohols, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose,regenerated cellulose, polyvinyl pyrollidone, tetrafluoroethylene,polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM),sulfonated EPDM, styrene butadiene rubber, fluoro rubber and variouscopolymers.

The filler is an optional ingredient used to inhibit cathode expansion.There is no particular limit to the filler, so long as it does not causechemical changes in the fabricated battery and is a fibrous material. Asexamples of the filler, there may be used olefin polymers such aspolyethylene and polypropylene; and fibrous materials such as glassfiber and carbon fiber.

The anode is fabricated by applying an anode active material to an anodecurrent collector, followed by drying. If necessary, other components asdescribed above may be further included.

The anode current collector is generally fabricated to have a thicknessof 3 to 500 μm. There is no particular limit to materials for the anodecurrent collector, so long as they have suitable conductivity withoutcausing chemical changes in the fabricated battery. As examples ofmaterials for the anode current collector, mention may be made ofcopper, stainless steel, aluminum, nickel, titanium, sintered carbon,copper or stainless steel having a surface treated with carbon, nickel,titanium or silver, and aluminum-cadmium alloys. Similar to the cathodecurrent collector, the anode current collector may also be processed toform fine irregularities on the surfaces thereof so as to enhanceadhesion to the anode active material. In addition, the anode currentcollector may be used in various forms including films, sheets, foils,nets, porous structures, foams and non-woven fabrics.

As examples of the anode active materials utilizable in the presentinvention, mention may be made of carbon such as non-graphitizing carbonand graphite-based carbon; metal composite oxides such asLi_(y)Fe₂O₃(0≦y≦1), Li_(y)WO₂ (0≦y≦1) and Sn_(x)Me_(1−x)Me′_(y)O_(z)(Me: Mn, Fe, Pb or Ge; Me′: Al, B, P, Si, Group I, Group II and GroupIII elements of the Periodic Table of the Elements, or halogens; 0<x≦1;1≦y≦3; and 1≦z≦8); lithium metals; lithium alloys; silicon-based alloys;tin-based alloys; metal oxides such as SnO, SnO₂, PbO, PbO₂, Pb₂O₃,Pb₃O₄, Sb₂O₃, Sb₂O₄, Sb₂O₅, GeO, GeO₂, Bi₂O₃, Bi₂O₄, and Bi₂O₅;conductive polyacetylene; and Li—Co—Ni based materials.

The separator is interposed between the cathode and anode. As theseparator, an insulating thin film having high ion permeability andmechanical strength is used. The separator typically has a pore diameterof 0.01 to 10 μm and a thickness of 5 to 300 μm. As the separator,sheets or non-woven fabrics made of an olefin polymer such aspolypropylene and/or glass fibers or polyethylene, which have chemicalresistance and hydrophobicity, are used. When a solid electrolyte suchas a polymer is employed as the electrolyte, the solid electrolyte mayalso serve as both the separator and electrolyte.

The lithium salt-containing non-aqueous electrolyte is composed of anelectrolyte and a lithium salt. As the electrolyte, a non-aqueousorganic solvent, an organic solid electrolyte, or an inorganic solidelectrolyte may be utilized.

As the non-aqueous organic solvent that can be used in the presentinvention, for example, mention may be made of aprotic organic solventssuch as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate,butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxy ethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide,dimethylformamide, dioxolane, acetonitrile, nitromethane, methylformate, methyl acetate, phosphoric acid triester, trimethoxy methane,dioxolane derivatives, sulfolane, methyl sulfolane,1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives,tetrahydrofuran derivatives, ether, methyl propionate, and ethylpropionate.

As examples of the organic solid electrolyte utilized in the presentinvention, mention may be made of polyethylene derivatives, polyethyleneoxide derivatives, polypropylene oxide derivatives, phosphoric acidester polymers, alginate/poly-1-lysine, polyester sulfide, polyvinylalcohols, polyvinylidene fluoride, and polymers containing ionicdissociation groups.

As examples of the inorganic solid electrolyte utilized in the presentinvention, mention may be made of nitrides, halides and sulfates oflithium such as Li₃N, LiI, Li₅NI₂, Li₃N—LiI—LiOH, LiSiO₄,LiSiO₄—LiI—LiOH, Li₂SiS₃, Li₄SiO₄, Li₄SiO₄—LiI—LiOH, andLi₃PO₄—Li₂S—SiS₂.

The lithium salt is a material that is readily soluble in theabove-mentioned non-aqueous electrolyte and may include, for example,LiCl, LiBr, LiI, LiClO₄, LiBF₄, LiB₁₀Cl₁₀, LiPF₆, LiCF₃SO₃, LiCF₃CO₂,LiAsF₆, LiSbF₆, LiAlCl₄, CH₃SO₃Li, CF₃SO₃Li, (CF₃SO₂)₂NLi, chloroboranelithium, lower aliphatic carboxylic acid lithium, lithium tetraphenylborate, and imide.

Additionally, in order to improve charge/discharge characteristics andflame retardancy, for example, pyridine, triethylphosphite,triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphorictriamide, nitrobenzene derivatives, sulfur, quinone imine dyes,N-substituted oxazolidinone, N,N-substituted imidazolidine, ethyleneglycol dialkyl ether, ammonium salts, pyrrole, 2-methoxy ethanol,aluminum trichloride, or the like may be added to the non-aqueouselectrolyte. If necessary, in order to impart incombustibility, thenon-aqueous electrolyte may further include halogen-containing solventssuch as carbon tetrachloride and ethylene trifluoride. Further, in orderto improve high-temperature storage characteristics, the non-aqueouselectrolyte may additionally include carbon dioxide gas.

In accordance with yet another aspect of the present invention, there isprovided a lithium mixed transition metal oxide comprising a mixedtransition metal of Ni, Mn and Co and comprising mixed-transition metaloxide layers (“MO layers”) containing Ni ions, and reversible lithiumlayers which allow intercalation and deintercalation of lithium ions,wherein the MO layers and the reversible lithium layers are disposedalternately and repeatedly to form a layered crystal structure, and aportion of Ni ions derived from the MO layer are inserted into thereversible lithium layers, thereby resulting in interconnection of theMO layers.

Generally, the use of nickel-based lithium mixed transition metal oxideas the cathode active material may bring about the collapse of thecrystal structure upon intercalation/deintercalation of lithium ions.However, the lithium mixed transition metal oxide in accordance with thepresent invention exhibits stabilization of the crystal structure due tonickel ions inserted into the reversible lithium layers and thereforedoes not undergo additional structural collapse that may result fromoxygen desorption, thereby simultaneously improving the lifespancharacteristics and safety.

Further, the present invention provides a lithium mixed transition metaloxide having a composition represented by Formula II below, wherein amolar cation mixing ratio for a portion of Ni located in lithium sitesof the lithium mixed transition metal oxide is 0.03 to 0.07 of the totalcontent of Ni, and is sufficient to provide the structurally stable,layered crystal structure.

Li_(x′)M″_(y′)A′_(z′)O₂   (II)

wherein

0.95≦x′≦1.05, 0≦z′<0.05, x′+y′≈2, and y′+z′=1;

M″ is a mixed transition metal of Ni, Mn and Co; and

A′ is at least one element selected from the group consisting of B, Ca,Zr, S, F, P, Bi, Al, Mg, Zn, Sr, Cu, Fe, Ga, In, Cr, Ge and Sn.

According to the experimental results by the present inventors, thecation mixing ratio is particularly preferably 0.03 to 0.07, uponconsidering of both the charge and cycle stability, and the ratecharacteristics. When the cation mixing ratio is within theabove-specified range, the crystal structure can be further stablymaintained and a hindrance of Li⁺ migration can be minimized.

Further, the present invention provides a lithium mixed transition metaloxide having a composition of Formula II, wherein a molar ratio oflithium to a mixed transition metal (M) is 0.95:1 to 1.04:1, and aportion of Ni ions are inserted into lithium sites in the reversiblelithium layers to interconnect the MO layers and the reversible lithiumlayers and to provide a structurally stable layered crystal structure.

Therefore, there is no need to add an excess of a lithium source and itis also possible to significantly reduce the possibility of retention ofthe water-soluble base impurities which may derive from the lithiumsource.

When the molar ratio of lithium to the mixed transition metal (M) is0.95:1 to 1.04:1, this may lead to appropriate cation mixing to therebyfurther enhance the structural stability and minimize a hindrance of Li⁺migration during the charge and discharge process. Therefore, thelithium mixed transition metal oxide exhibits excellent stability,lifespan characteristics and rate characteristics. Further, this metaloxide has a high capacity, as well as high chemical resistance andhigh-temperature stability due to substantially no water-soluble bases.

In one preferred embodiment, the lithium mixed transition metal oxidehas a composition of Formula II and a cation mixing ratio for the amountof Ni ions located in lithium sites is 0.03 to 0.07 based on the totalcontent of Ni to thereby stably support the layered crystal structure.Further, the present invention provides a lithium mixed transition metaloxide comprising mixed transition metal oxide layers (“MO layers”)comprising Ni, Mn and Co, and reversible lithium layers which allowlithium ion intercalation/deintercalation, wherein the MO layers and thereversible lithium layers are disposed alternately and repeatedly toform a layered crystal structure, the MO layers contain Ni³⁺ ions andNi²⁺ ions, and a portion of Ni²⁺ ions derived from the MO layers areinserted into the reversible lithium layers.

Such a structure is not known in the art and makes a significantcontribution to the stability and lifespan characteristics of thelithium mixed transition metal oxide disclosed herein.

Further, the present invention provides a lithium mixed transition metaloxide having a composition of Formula I and comprising mixed transitionmetal oxide layers (“MO layers”), wherein the MO layers contain Ni³⁺ions and Ni²⁺ ions, and a portion of Ni²⁺ ions derived from the MOlayers are inserted into the reversible lithium layers, duringpreparation of the lithium mixed transition metal oxide by a reaction oflithium mixed transition metal oxide precursor materials under anO₂-deficient atmosphere.

In comparison therefore, a conventional lithium transition metal oxidehas a crystal structure with no insertion of Ni²⁺ ions into the MO layeras effected by the reaction of identical precursor materials under anoxygen atmosphere, whereas the lithium mixed transition metal oxide inaccordance with the present invention has a structure wherein relativelylarge amounts of Ni²⁺ ions are produced by carrying out the reactionunder an oxygen-deficient atmosphere and a portion of the thus-producedNi²⁺ ions are inserted into the reversible lithium layers. Therefore, asillustrated hereinbefore, the crystal structure is stably maintained tothereby exert superior sintering stability, and a secondary batterycomprising the same exhibits excellent cycle stability.

EXAMPLES

Now, the present invention will be described in more detail withreference to the following Examples. These examples are provided onlyfor illustrating the present invention and should not be construed aslimiting the scope and spirit of the present invention.

Example 1

A mixed hydroxide of Formula MOOH(M=Ni_(4/15)(Mn_(1/2)Ni_(1/2))_(8/15)Co_(0.2)) as a mixed transitionmetal precursor and Li₂CO₃ were mixed in a stoichiometric ratio(Li:M=1.02:1), and the mixture was sintered in air at varioustemperatures of 850(Ex. 1A), 900 (Ex. 1B), 950 (Ex. 1C), and 1,000° C.for 10 hours, to preparing a lithium mixed transition metal oxide.Herein, secondary particles were maintained intact without beingcollapsed, and the crystal size increased with an increase in thesintering temperature.

X-ray analysis showed that all samples have a well-layered crystalstructure. Further, a unit cell volume did not exhibit a significantchange with an increase in the sintering temperature, thus representingthat there was no significant oxygen-deficiency and no significantincrease of cation mixing, in conjunction with essentially no occurrenceof lithium evaporation.

The crystallographic data for the thus-prepared lithium mixed transitionmetal oxide are given in Table 1 below, and FESEM images thereof areshown in FIG. 5. From these results, it was found that the lithium mixedtransition metal oxide is LiNiMO₂ having a well-layered crystalstructure with the insertion of nickel at a level of 3.9 to 4.5% into areversible lithium layer. Further, it was also found that even thoughLi₂CO₃ was used as a raw material and sintering was carried out in air,proper amounts of Ni²⁺ ions were inserted into the lithium layer,thereby achieving the desired structural stability.

Particularly, Sample B, sintered at 900° C., exhibited a high c:a ratioand therefore excellent crystallinity, a low unit cell volume and areasonable cation mixing ratio. As a result, Sample B showed the mostexcellent electrochemical properties, and a BET surface area of about0.4 to about 0.8 m²/g.

TABLE 1 Example 1(A-D) (A) (B) (C) (D) Sintering temp. 850° C. 900° C.950° C. 1,000° C. Unit cell vol. 33.902 Å³ 33.921 Å³ 33.934 Å³ 33.957 Å³Normalized c:a ratio 1.0123 1.0122 1.0120 1.0118 c:a/24{circumflex over( )}0.5 Cation mixing 4.5% 3.9% 4.3% 4.5% (Rietveld refinement)

Comparative Example 1

50 g of a commercial sample having a composition ofLiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ represented by Formula LiNi_(1−x)M_(x)O₂(x=0.3, and M=Mn_(1/3)Ni_(1/3)Co_(1/3)) was heated in air to 750° C.(CEx. 1A), 850° C. (CEx. 1B), 900° C. (CEx. 1C), and 950° C. (CEx. 1D)(10 hrs), respectively.

X-ray analysis was carried out to obtain detailed lattice parameterswith high resolution. Cation mixing was observed by Rietveld refinement,and morphology was analyzed by FESEM. The results thus obtained aregiven in Table 2 below. Referring to Table 2, it can be seen that all ofthe samples heated to a temperature of T≧750° C. (CEx. 1A-D) exhibitedcontinuous degradation of a crystal structure (increased cation mixing,increased lattice constant and decreased c:a ratio). FIG. 6 shows aFESEM image of a commercial sample as received and a FESEM image of thesame sample heated to 850° C. (CEx. 1B) in air; and it can be seen thatthe sample heated to a temperature of T≧850° C. (CEx. 1B-D) exhibitedstructural collapse. This is believed to be due to that Li₂CO₃, formedduring heating in air, melts to thereby segregate particles.

TABLE 2 Comp. Ex. 1 (A-D) (A) (B) (C) (D) Sintering temp. 750° C. 850°C. 900° C. 950° C. Unit cell vol. 33.902 Å³ 33.920 Å³ 33.934 Å³ 33.957Å³ Normalized c:a ratio 1.0103 1.0100 1.0090 1.0085 c:a/24{circumflexover ( )}0.5 Cation mixing 10% 12% 15% 18% (Rietveld refinement)

Therefore, it can be seen that it is impossible to produce aconventional lithium mixed transition metal oxide having theabove-specified composition in the air containing trace amounts ofcarbon dioxide, due to thermodynamic limitations. In addition, uponproducing the lithium transition metal oxide having the abovecomposition according to a conventional method, the use of Li₂CO₃ as araw material is accompanied by evolution of CO₂ due to decomposition ofLi₂CO₃, thereby leading to thermodynamic hindrance of additionaldecomposition of Li₂CO₃, consequently resulting in no furtherprogression of the reaction. For these reasons, it was shown that such aconventional method cannot be applied to the practical productionprocess.

Comparative Example 2

The pH titration was carried out at a flow rate of >2 L/min for 400 g ofa commercial sample having a composition of LiNi_(0.8)Co_(0.2)O₂. Theresults thus obtained are given in FIG. 7. In FIG. 7, Curve A (CEx. 2A)represents pH titration for the sample as received, and Curve B (CEx.2B) represents pH titration for the sample heated to 800° C. in a flowof pure oxygen for 24 hours. From the analysis results of pH profiles,it can be seen that the contents of Li₂CO₃ before and after heattreatment were the same therebetween, and there was no reaction ofLi₂CO₃ impurities. That is, it can be seen that the heat treatment underan oxygen atmosphere resulted in no additional production of Li₂CO₃impurities, but Li₂CO₃ impurities present in the particles were notdecomposed. Through slightly increased cation mixing, a slightlydecreased c:a ratio and a slightly decreased unit cell volume from theX-ray analysis results, it was found that the content of Li slightlydecreased in the crystal structure of LiNiO₂ in conjunction with theformation of a small amount of Li₂O. Therefore, it can be seen that itis impossible to prepare a stoichiometric lithium mixed transition metaloxide with no impurities and no lithium-deficiency in a flow of oxygengas or synthetic air.

Comparative Example 3

LiAl_(0.02)Ni_(0.78)Co_(0.2)O₂ containing less than 3% aluminumcompound, as commercially available Al/Ba-modified, high-nickel LiNiO₂,was stored in a wet chamber (90% relative humidity, abbreviated “RH”) at60° C. in air. The pH titration was carried out for a sample prior toexposure to moisture, and samples wet-stored for 17 hrs and 3 days,respectively. The results thus obtained are given in FIG. 8. Referringto FIG. 8, an amount of water-soluble bases was low before storage, butsubstantial amounts of water-soluble bases, primarily comprising Li₂CO₃,were continuously formed upon exposure to air. Therefore, even when aninitial amount of Li₂CO₃ impurities was low, it was revealed that thecommercially available high-nickel LiNiO₂ is not stable in air andtherefore rapidly decomposes at a substantial rate, and substantialamounts of Li₂CO₃ impurities are formed during storage.

Example 2

The pH titration was carried out for a sample of the lithium mixedtransition metal oxide in accordance with Example 2 prior to exposure tomoisture, and samples stored in a wet chamber (90% RH) at 60° C. in airfor 17 hours and 3 days, respectively. The results thus obtained aregiven in FIG. 9. Upon comparing the lithium mixed transition metal oxideof Example 2 (see FIG. 9) with the sample of Comparative Example 3 (seeFIG. 8), the sample of Comparative Example 3 (stored for 17 hours)exhibited consumption of about 20 mL of HCl, whereas the sample ofExample 2 (stored for 17 hours) exhibited consumption of 10 mL of HCl,thus showing an about two-fold decrease in production of thewater-soluble bases. Further, in 3-day-storage samples, the sample ofComparative Example 3 exhibited consumption of about 110 mL of HCl,whereas the sample of Example 2 exhibited consumption of 26 mL of HCl,which corresponds to an about five-fold decrease in production of thewater-soluble bases. Therefore, it can be seen that the sample ofExample 2 decomposed at a rate about five-fold slower than that of thesample of Comparative Example 3. Then, it can be shown that the lithiummixed transition metal oxide of Example 2 exhibits superior chemicalresistance even when it is exposed to air and moisture.

Comparative Example 4

A high-nickel LiNiO₂ sample having a composition ofLiNi_(0.8)Mn_(0.05)Co_(0.15)O₂, as a commercial sample which wassurface-coated with AlPO₄ followed by gentle heat treatment, wassubjected to pH titration before and after storage in a wet chamber. Asa result of pH titration, 12 mL of 0.1M HCl was consumed per 10 gcathode, an initial content of Li₂CO₃ was low, and the content of Li₂CO₃after storage was slightly lower (80 to 90%) as compared to the sampleof Comparative Example 3, but a higher content of Li₂CO₃ was formed thanin the sample of Example 2. Consequently, it was found that theaforementioned high-Ni LiNiO₂ shows no improvements in the stabilityagainst exposure to the air even when it was surface-coated, and alsoexhibits insignificant improvements in the electrochemical propertiessuch as the cycle stability and rate characteristics.

Example 3

A mixture of Li₂CO₃ with mixed hydroxide of Formula MOOH(M=Ni_(4/15)(Mn_(1/2)Ni_(1/2))_(8/15)Co_(0.2)) was introduced into afurnace with an about 20 L chamber and sintered at 920° C. for 10 hours,during which more than 10 m³ of air was pumped into the furnace, therebypreparing about 5 kg of LiNiMO₂ in one batch.

After sintering was complete, a unit cell constant was determined byX-ray analysis, and a unit cell volume was compared with a target value(Sample B of Example 1: 33.921 Å³). ICP analysis showed that a ratio ofLi and M is very close to 1.00, and the unit cell volume was within thetarget range. FIG. 10 shows an SEM image of the thus-prepared cathodeactive material and FIG. 11 shows results of Rietveld refinement.Referring to these drawings, it was found that the sample exhibits highcrystallinity and well-layered structure, a mole fraction of Ni²⁺inserted into a reversible lithium layer is 3.97%, and the calculatedvalue and the measured value of the mole fraction of Ni²⁺ areapproximately the same.

Meanwhile, upon performing pH titration, less than 10 mL of 0.1M HCl wasconsumed to titrate 10 g of a cathode to achieve a pH of less than 5,which corresponds to a Li₂CO₃ impurity content of less than about 0.035wt %. Hence, these results show that it is possible to achieve massproduction of Li₂CO₃-free LiNiMO₂ having a stable crystal structure fromthe mixed hydroxide and Li₂CO₃ by a solid-state reaction.

Experimental Example 1 Test of Electrochemical Properties

Coin cells were fabricated using the lithium mixed transition metaloxide of Example 3, LiNiMO₂ of Comparative Examples 2 to 4, andcommercial LiMO₂ with M=(Ni_(1/2)Mn_(1/2))_(1−x)Co_(x) and x=0.17(Comparative Example 5) and x=0.33 (Comparative Example 6),respectively, as a cathode, and a lithium metal as an anode.Electrochemical properties of the thus-fabricated coin cells weretested. Cycling was carried out at 25° C. and 60° C., primarily a chargerate of C/5 and a discharge rate of C/5 (1 C=150 mA/g) within a range of3 to 4.3 V.

Experimental results of the electrochemical properties for the coincells of Comparative Examples 2 to 4 are given in Table 3 below.Referring to Table 3, the cycle stability was poor with the exception ofComparative Example 3 (Sample B). It is believed that ComparativeExample 4 (Sample C) exhibits the poor cycle stability due to thelithium-deficiency of the surface. Whereas, even though ComparativeExample 2 (Sample A) and Comparative Example 3 (Sample B) were notlithium-deficient, only Comparative Example 4 (Sample C) exhibited a lowcontent of Li₂CO₃. The presence of such Li₂CO₃ may lead to gas evolutionand gradual degradation of the performance (at 4.3 V, Li₂CO₃ slowlydecomposes with the collapse of crystals). That is, there are nonickel-based active materials meeting both the excellent cycle stabilityand the low-impurity content, and therefore it can be shown that theconventional nickel-based active materials suffer from poor cyclestability and low stability against exposure to air, in conjunction witha high level of Li₂CO₃ impurities and high production costs.

TABLE 3 Examples Sample A Sample B Sample C LiNi_(0.8)Co_(0.2)O₂Al/Ba-modified AlPO₄-coated Substrate Comp. Ex. 2 Comp. Ex. 3 Comp. Ex.4 Stoichiometric Stoichiometric Stoichiometric Li-deficient at Li:Msurfaces Li₂CO₃ impurities High High Low Capacity at 25° C. 193, 175mAh/g 195, 175 mAh/g 185, 155 mAh/g C/10, C/1 Capacity loss 30% per 100cycles 11% per 100 cycles >30% per 100 cycles

On the other hand, the cells of Comparative Examples 5 and 6 exhibited acrystallographic density of 4.7 and 4.76 g/cm³, respectively, which werealmost the same, and showed a discharge capacity of 157 to 159 mAh/g ata C/10 rate (3 to 4.3 V). Upon comparing with LiCoO₂ having acrystallographic density of 5.04 g/cm³ and a discharge capacity of 157mAh/g, a volume capacity of the cell of Comparative Example 5 is equalto a 93% level of LiCoO₂, and the cell of Comparative Example 6 exhibitsa crystallographic density corresponding to a 94% level of LiCoO₂.Therefore, it can be seen that a low content of Ni results in a poorvolume capacity.

Whereas, a crystallographic density of LiNiMO₂ in accordance withExample 3 was 4.74 g/cm³ (cf. LiCoO₂: 5.05 g/cm³). A discharge capacitywas more than 170 mAh/g (cf. LiCoO₂: 157 mAh/g) at C/20, thusrepresenting that the volume capacity of LiNiMO₂ was much improved ascompared to LiCoO₂.

Table 4 below summarizes electrochemical results of coin cells usingLiNiMO₂ in accordance with Example 3 as a cathode, and FIG. 12 depictsvoltage profiles, discharge curves and cycle stability.

TABLE 4 Capacity retention Primary after 100 cycles charge(extrapolated) capacity C/5-C/5 cycle, 3.0-4.3 V, Discharge capacity3.0-4.3 V C/10 25° C., 25° C., 60° C., 25° C. 60° C. — C/1 C/20C/20 >96% >90% >190 mAh/g 152 mA/g 173 mAh/g 185 mAh/g

Experimental Example 2 Determination of Thermal Stability

In order to examine the thermal stability for the lithium mixedtransition metal oxide of Example 3 and LiNiMO₂ in accordance withComparative Examples 3 and 4, DSC analysis was carried out. Thethus-obtained results are given in FIGS. 13 and 14. For this purpose,coin cells (anode: lithium metal) were charged to 4.3 V, disassembled,and inserted into hermetically sealed DSC cans, followed by injection ofan electrolyte. A total weight of the cathode was about 50 to about 60mg, a total weight of the electrolyte was approximately the same.Therefore, an exothermic reaction is strongly cathode-limited. The DSCmeasurement was carried out at a heating rate of 0.5 K/min.

As a result, Comparative Example 3 (Al/Ba-modified LiNiO₂) andComparative Example 4 (AlPO₄-coated LiNiO₂) showed the initiation of astrong exothermic reaction at a relatively low temperature.Particularly, Comparative Example 3 exhibited a heat evolution thatexceeds the limit of the device. The total accumulation of heatgeneration was large, i.e. well above 2,000 kJ/g, thus indicating a lowthermal stability (see FIG. 13).

Meanwhile, LiNiMO₂ of Example 3 in accordance with the present inventionexhibited a low total heat evolution, and the initiation of anexothermic reaction at a relatively high temperature (about 260° C.) ascompared to Comparative Examples 3 and 4 (about 200° C.) (see FIG. 14).Therefore, it can be seen that the thermal stability of LiNiMO₂ inaccordance with the present invention is excellent.

Experimental Example 3 Test of Electrochemical Properties of PolymerCells with Application of Lithium Mixed Transition Metal Oxide

Using the lithium mixed transition metal oxide of Example 3 as a cathodeactive material, a pilot plant polymer cell of 383562 type wasfabricated. For this purpose, the cathode was mixed with 17% by weightLiCoO₂, and the cathode slurry was NMP/PVDF-based slurry. No additivesfor the purpose of preventing gelation were added. The anode was amesocarbon microbead (MCMB) anode. The electrolyte was a standardcommercial electrolyte free of additives known to reduce excessiveswelling. Experiments were carried out at 60° C. and charge anddischarge rates of C/5. A charge voltage was from 3.0 to 4.3 V.

FIG. 15 shows the cycle stability of the battery of the presentinvention (0.8 C charge, 1C discharge, 3 to 4 V, 2 V) at 25° C. Anexceptional cycle stability (91% at C/1 rate after 300 cycles) wasachieved at room temperature. The impedance build up was low. Also, thegas evolution during storage was measured. The result thus obtained aregiven in FIG. 16. During a 4 h-90° C. fully charged (4.2 V) storage, avery small amount of gas was evolved and only a small increase ofthickness was observed. The increase of thickness was within or lessthan the value expected for good LiCoO₂ cathodes tested in similar cellsunder similar conditions. Therefore, it can be seen that LiNiMO₂ inaccordance with the present invention exhibits very high stability andchemical resistance.

Experimental Example 4

Samples with different Li:M molar ratios were prepared from Formula MOOH(M=Ni_(4/15)(Mn_(1/2)N_(1/2))_(8/15)Co_(0.2)). Li₂CO₃ was used as alithium source. 7 samples each of about 50 g with Li:M ratios rangingfrom 0.925 to 1.125 were prepared by a sintering process in air at atemperature of 910 to 920° C. Then, electrochemical properties weretested.

Table 5 below provides the obtained crystallographic data. The unit cellvolume changes smoothly according to the Li:M ratio. FIG. 17 shows itscrystallographic map. All samples are located on a straight line.According to the results of pH titration, the content of soluble baseincreases slightly with an increase of the Li:M ratio. The soluble baseprobably originates from the surface basicity (ion exchange) but notfrom the dissolution of Li₂CO₃ impurities as observed in ComparativeExample 1.

Therefore, this experiment clearly shows that the lithium mixedtransition metal oxide prepared by the method in accordance with thepresent invention is in the Li stoichiometric range and additional Li isinserted into the crystal structure. In addition, it can be seen thatstoichiometric samples without Li₂CO₃ impurity can be obtained even whenLi₂CO₃ is used as a precursor and the sintering is carried out in air.

That is, as the Li/M ratio decreases, the amount of Ni²⁺ inserted intothe reversible lithium layer gradually increases. Insertion ofexcessively large amounts of Ni²⁺ into the reversible lithium layerhinders the movement of Li⁺ during the charge/discharge process, therebyresulting in decreased capacity or poor rate characteristics. On theother hand, if the Li/M ratio is excessively high, the amount of Ni²⁺inserted into the reversible lithium layer is too low, which may resultin structural instability leading to deterioration of the battery safetyand lifespan characteristics. Further, at the high Li/M value, amountsof unreacted Li₂CO₃ increase to thereby result in a high pH-titrationvalue. Therefore, upon considering the performance and safety of thebattery, the ratio of Li:M is particularly preferably in a range of 0.95to 1.04 (Samples B, C and D) to ensure that the value of Ni²⁺ insertedinto the lithium layer is in a range of 3 to 7%.

TABLE 5 Samples A B C D E F G Li:M ratio 0.925 0.975 1.0 1.025 1.051.075 1.125 Unit cell vol. 34.111 Å³ 34.023 Å³ 33.923 Å³ 33.921 Å³33.882 Å³ 33.857 Å³ 33.764 Å³ c:a ratio 1.0116 1.0117 1.0119 1.01221.0122 1.0123 1.0125 Cation mixing 8.8% 6.6% 4.7% 4.0% 2.1% 2.5% 1.4% pHtitration 3 3.5 5 9 15 19 25

Example 4

A mixed hydroxide of Formula MOOH(M=Ni_(4/15)(Mn_(1/2)Ni_(1/2))_(8/15)Co_(0.2)) as a mixed transitionmetal precursor and Li₂CO₃ were mixed in a molar ratio of Li:M=1.01:1,and the mixture was sintered in air at 900° C. for 10 hours, therebypreparing 50 g of a lithium mixed transition metal oxide having acomposition of LiNi_(0.53)Co_(0.2)Mn_(0.27)O₂.

X-ray analysis was carried out to obtain detailed lattice parameterswith high resolution. Cation mixing was observed by Rietveld refinement.The results thus obtained are given in Table 6 below.

Comparative Example 7

A lithium transition metal oxide was prepared in the same manner as inExample 4, except that a molar ratio of Li:M was set to 1:1 andsintering was carried out under an O₂ atmosphere. Then, X-ray analysiswas carried out and the cation mixing was observed. The results thusobtained are given in Table 6 below.

TABLE 6 Ex. 4 Comp. Ex. 7 Li:M 1.01:1 1:1 Unit cell vol. 33.921 Å³33.798 Å³ Normalized c:a ratio 1.0122 1.0124 c:a/24{circumflex over( )}0.5 Cation mixing 4.6% 1.5%

As can be seen from Table 6, the lithium transition metal oxide ofComparative Example 7 exhibited a significantly low cation mixing ratiodue to the heat treatment under the oxygen atmosphere. This case suffersfrom deterioration of the structural stability. That is, it can be seenthat the heat treatment under the oxygen atmosphere resulted in thedevelopment of a layered structure due to excessively low cation mixing,but migration of Ni²⁺ ions was hindered to an extent that the cyclestability of the battery is arrested.

Example 5

A lithium mixed transition metal oxide having a composition ofLiNi_(0.4)Co_(0.3)Mn_(0.3)O₂ was prepared in the same manner as inExample 4, except that a mixed hydroxide of Formula MOOH(M=Ni_(1/10)(Mn_(1/2)Ni_(1/2))_(6/10)Co_(0.3)) was used as a mixedtransition metal precursor, and the mixed hydroxide and Li₂CO₃ weremixed in a ratio of Li:M=1:1. The cation mixing was observed by X-rayanalysis and Rietveld refinement. The results thus obtained are given inTable 7 below.

TABLE 7 Li:M 1:1 Unit cell vol. 33.895 Å³ Normalized c:a ratio 1.0123c:a/24{circumflex over ( )}0.5 Cation mixing 3% Capacity (mAh/g) 155

Example 6

A lithium mixed transition metal oxide having a composition ofLiNi_(0.65)Co_(0.2)Mn_(0.15)O₂ was prepared in the same manner as inExample 4, except that a mixed hydroxide of Formula MOOH(M=Ni_(5/10)(Mn_(1/2)Ni_(1/2))_(3/10)Co_(0.2)) was used as a mixedtransition metal precursor, and the mixed hydroxide and Li₂CO₃ weremixed in a molar ratio of Li:M=1:1. The cation mixing was observed byX-ray analysis and Rietveld refinement. The results thus obtained aregiven in Table 8 below.

TABLE 8 Li :M 1:1 Unit cell vol. 34.025 Å³ Normalized c:a ratio 1.0107c:a/24{circumflex over ( )}0.5 Cation mixing 7% Capacity (mAh/g) 172

From the results shown in Tables 7 and 8, it can be seen that thelithium mixed transition metal oxide in accordance with the presentinvention provides desired effects, as discussed hereinbefore, in agiven range.

INDUSTRIAL APPLICABILITY

As apparent from the above description, a lithium mixed transition metaloxide in accordance with the present invention has a specifiedcomposition and interconnection of MO layers via the insertion of aportion of MO layer-derived Ni ions into reversible lithium layers,whereby the crystal structure can be stably supported to thereby preventthe deterioration of cycle characteristics. In addition, such a lithiummixed transition metal oxide exhibits a high battery capacity and issubstantially free of impurities such as water-soluble bases, therebyproviding excellent storage stability, decreased gas evolution andconsequently superior high-temperature stability.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A lithium mixed transition metal oxide having a compositionrepresented by Formula I and comprising mixed transition metal oxide(MO) layers comprising Ni ions, and reversible lithium layers whichallow intercalation and deintercalation of lithium ions, wherein the MOlayers and the reversible lithium layers are disposed alternately andrepeatedly to form a layered crystal structure, and a portion of Ni ionsderived from the MO layer are inserted into reversible lithium layers tointerconnect the MO layers and the reversible lithium layers:Li_(x)M_(y)O₂   (I) wherein: M=M′_(1−k)A_(k), wherein M′ isNi_(1−a−b)(Ni_(1/2)Mn_(1/2))_(a)Co_(b), 0.65≦a+b≦0.85 and 0.1≦b≦0.4; Ais a dopant; 0≦k<0.05; and x+y≈2 and 0.95≦x≦1.05.
 2. The lithium mixedtransition metal oxide according to claim 1, wherein the Ni ionscomprise Ni²⁺ ions and Ni³⁺ ions coexisting in the MO layers, and theportion of Ni²⁺ ions are inserted and bound into the reversible lithiumlayers.
 3. The lithium mixed transition metal oxide according to claim1, wherein in Formula I a mole fraction of Ni is 0.4 to 0.7, a molefraction of Mn is 0.05 to 0.4, and a mole fraction of Co is 0.1 to 0.4.4. The lithium mixed transition metal oxide according to claim 2,wherein the mole fraction of Ni²⁺ ions is 0.05 to 0.4, based on thetotal content of Ni ions.
 5. The lithium mixed transition metal oxideaccording to claim 4, wherein the portion as a mole fraction of Ni²⁺ions inserted and bound into the reversible lithium layer is 0.03 to0.07, based on the total content of Ni.
 6. The lithium mixed transitionmetal oxide according to claim 1, wherein the molar ratio of Li:M is0.95 to 1.04:1.
 7. The lithium mixed transition metal oxide according toclaim 1, wherein the lithium mixed transition metal oxide issubstantially free of Li₂CO₃ as impurities.
 8. The lithium mixedtransition metal oxide according to claim 7, wherein Li₂CO₃ is containedin an amount such that less than 20 mL of a 0.1 M HCl titrant solutionis added during pH titration of a solution of water-soluble basesextracted from the lithium mixed transition metal oxide to reach a valueof less than 5, wherein the solution of the water-soluble bases isprepared by repeatedly soaking and decanting 10 g of the lithium mixedtransition metal oxide with water such that the resulting solutioncontains all of the water soluble bases of the lithium mixed transitionmetal oxide, and wherein the total volume of the solution ofwater-soluble bases is 200 mL.
 9. The lithium mixed transition metaloxide according to claim 8, wherein the amount of the 0.1M HCl solutionadded during pH titration to reach the pH of less than 5 is less than 10mL.
 10. A cathode active material for a secondary battery, comprisingthe lithium mixed transition metal oxide of claim
 1. 11. A lithiumsecondary battery comprising the cathode active material of claim 10.12. A lithium mixed transition metal oxide comprising mixed transitionmetals of Ni, Mn and Co, and comprising mixed transition metal oxide(MO) layers containing Ni ions, and reversible lithium layers whichallow intercalation and deintercalation of lithium ions, wherein the MOlayers and the reversible lithium layers are disposed alternately andrepeatedly to form a layered crystal structure, and a portion of Ni ionsderived from the MO layers are inserted into the reversible lithiumlayers to interconnect the MO layers.
 13. A lithium mixed transitionmetal oxide having a composition represented by Formula II, wherein amolar cation mixing ratio for a portion of Ni located in lithium sitesof the lithium mixed transition metal oxide is 0.03 to 0.07 of the totalcontent of Ni, and is sufficient to provide a structurally stablelayered crystal structure:Li_(x′)M″_(y′)A′_(z′)O₂   (II) wherein 0.95≦x′≦1.05, 0≦z′<0.05, x′+y′≈2,and y′+z′=1; M″ is a mixed transition metal of Ni, Mn and Co; and A′ isat least one element selected from the group consisting of B, Ca, Zr, S,F, P, Bi, Al, Mg, Zn, Sr, Cu, Fe, Ga, In, Cr, Ge and Sn.
 14. A lithiummixed transition metal oxide having the composition of Formula II ofclaim 13, wherein a molar ratio of lithium to a mixed transition metal(M) is 0.95:1 to 1.04:1, and a portion of Ni ions derived from the MOlayer are inserted into the reversible lithium layers to interconnectthe MO layers and the reversible lithium layers.
 15. The lithium mixedtransition metal oxide according to claim 14, wherein the portion of Niions as a mole fraction inserted into the lithium sites is 0.03 to 0.07based on the total content of Ni.
 16. A lithium mixed transition metaloxide comprising mixed transition metal oxide (MO) layers of Ni, Mn andCo, and reversible lithium layers which allow intercalation anddeintercalation of lithium ions, wherein the MO layers and thereversible lithium layers are disposed alternately and repeatedly toform a layered crystal structure, the MO layers contain Ni³⁺ ions andNi²⁺ ions, and a portion of Ni²⁺ ions derived from the MO layers areinserted into the reversible lithium layers.
 17. A lithium mixedtransition metal oxide having a composition of Formula I of claim 1 andcomprising mixed transition metal oxide (MO) layers containing Ni ions,and reversible lithium layers which allow intercalation anddeintercalation lithium ions, wherein the MO layers contain Ni³⁺ ionsand Ni²⁺ ions, and a portion of Ni²⁺ ions derived from the MO layers areinserted into the reversible lithium layers during preparation of thelithium mixed transition metal oxide by a reaction of lithium mixedtransition metal oxide precursor materials under an O₂-deficientatmosphere.